WHAT IS GROWTH? “Developmental increase in mass.’’- Stewart.(1982) “An increase in size or number.” - Profitt. (1986) “Normal changes in amount of living substance.’’- Moyers(1988) “Growth signifies an increase, expansion or extension of any given tissue.” - Pinkham.(1994)
WHAT IS DEVELOPMENT?“Development is a progress towards maturity” – Todd(1931)“Development connotes a maturational process involving progressive differentiation at the cellular and tissue levels” – Enlow.
INTRODUCTION The human somatic cell contains 46 chromosomes, called as the diploid number. Out of which 44 are autosomes and the remaining 2 are sex chromosomes, designated as X and Y. The sex chromosomes in females are XX and in males are XY. There are two series of division of somatic cells- MITOSIS and MEIOSIS.
MITOSIS produces the same number of chromosomes in the resulting daughter cell while MEIOSIS produces half the number i.e. 23 designated as haploid, with resultant formation of gametes . Development begins with FERTILIZATION, the process in which the male gamete- the sperm, and the female gamete- the oocyte, unite to form a ZYGOTE.
THE PROCESS OF FERTILIZATIONTHE MALE GAMETE (SPERM) FUSES WITH THE FEMALE GAMETE (OVUM)
GROWTH IS BROADLY SUBDIVIDED AS:a. Prenatal growth 1. Period of ovum: From time of fertilization till 1 week. 2. Period of embryo: from 2nd week till 8th week 3. Period of fetus: from 9th week onwards till birthb. Postnatal growthc. Maturityd .Old age
PRENATAL GROWTH The fertilized ovum, undergoes cleavage as it moves toward the uterine cavity. The cells formed are called blastomeres, which soon begin to rearrange themselves in order to differentiate into various groups and layers. By the 4th day, when the zygote reaches the uterus, it is a many celled mass called a MORULA
As the cell mass divides, it enlarges and gains a fluid filled cavity termed the blastocele(5th day). The blastocoele separates the cell into 2 parts:-An outer cell layer, the trophoblast, and-An inner cell mass, the embryoblast.
IMPLANTATION : 6TH DAY The trophoblast attaches to the sticky endometrial surface on the posterior wall of the body of the uterus. The surface cells of the trophoblast produces enzymes that digest the uterine endometrial cells, which allows a deeper penetration of the cell mass.
During the second week, the cells of the inner cell mass of the growing blastocyst differentiate into 2 cell types: 1. Columnar shaped ectodermal cells and 2. Cuboidal shaped endodermal cells adjacent to blastocele. The amniotic cavity appears between the ectodermal cells and the overlying trophoblast.
Later in the developmental process, the amnion expands, filling the entire extra embryonic coelom . Thus in its final form, the amnion is a free membrane enclosing a fluid-filled space around the embryo. Again, cells grow from the trophoblast and the embryonic disc, to form a primitive yolk sac.
On day 15, a groove, called the primitive streak , appears on the surface of the midline of the dorsal aspect of the ectoderm of the embryonic disc. By day 16, a primitive knot of cells, the Henson’s node, appears at the cephalic end of the primitive streak. This knot gives rise to the cells that form the notochordal process.
Cells from the primitive streak and the notochordal process migrate laterally between the ectodermal and endodermal layers of the embryonic shield. These cells form the third germ cell layer called the mesodermal layer. By the end of the third week, the mesoderm migrates in a lateral direction between the ectoderm and the endoderm, except at the anterior prochordal plate and posterior cloacal membrane.
The anterior plate forms the future oropharyngeal membrane. Finally, mesodermal cells of the embryonic disc migrate peripherally to join the extra-embryonic mesoderm on the amnion and yolk sac. Anteriorly, mesodermal cells pass on either side of the prochordal plate to meet each other in front of this region.
FATE OF GERM LAYERS Ectodermal cells will give rise to the nervous system; the epidermis and its appendages (hair, nails, sebaceous and sweat glands); the epithelium lining the oral cavity, nasal cavities and sinuses; a part of the intraoral glands, and the enamel of the teeth. Endodermal cells will form the epithelial lining of the gastrointestinal tract and all associated organs. The mesoderm will give rise to the muscles and all the structures derived from the connective tissue(e.g., bone, cartilage, blood, dentin, pulp, cementum and the periodontal ligament). The embryonic disc will soon become altered by bends and folds necessary for further development.
POST NATAL GROWTH Pattern -Differential Growth -Cephalocaudal gradient of growth Variability Timing, rate & direction
PATTERN Pattern in growth represents proportionality .It refers not just to a set of proportional relationships at a point in time, but to a change in these proportional relationships over time. The physical arrangement of the body at any one time is a pattern of spatially proportioned parts.
DIFFERENTIAL GROWTH Different organs grow at different rates and at different times. Scammon’s curve of growth- by Richard Scammon. Lymphoid tissues attain a 200% growth by the age of ten and then regress afterwards. Neural tissue attains full growth by the age of six and then stops. General somatic tissues follow a sigmoid pattern. Genital tissue grow significantly only at puberty and achieve full growth at about 20 yrs of age.
CEPHALOCAUDAL GRADIENT OF GROWTH This means that there is an axis of increased growth extending from the head towards the feet. At about 3rd month of IU life, the head takes up 50% of total body length. By the time of birth, the proportion of head decreases to 30%. This proportion steadily declines till in adult, the proportion of head is only 12%.
VARIABILITY No two individuals with the exception of siamese twins are like. Hence it is important to have a “normal variability” before categorizing people as normal or abnormal
TIMING OF GROWTH One of the factors for variability in growth. Variations in timing arise, because the biologic clock of different individuals is different. It is influenced by: genetics sex related differences physique related environmental influences
GROWTH SPURTS Defined as periods of sudden growth acceleration Sex-linked Normal spurts are Just before birth 1 year after birth Infantile spurt – at 3 years age Mixed dentition growth spurt – 7-9 years (females); 8-11 years (males) Pre-pubertal spurt – 11-13 years(females); 14-16 years (males)
DEVELOPMENT OF OROPHARYNX The primitive oral cavity or stomodeum appears late in the third prenatal week as a pit or invagination of STOMODEUM the tissues underlying the forebrain.
This invagination appears as a result of the growth of the forebrain anteriorly and of the enlargement of the developing heart. At the deepest end of the stomodeum, the oral ectoderm lies in close contact with the foregut endoderm. The wall between the oral and pharyngeal cavity is termed the oropharyngeal membrane, as it separates the stomodeum from the first part of the foregut. During the fourth week of intrauterine life, the oropharyngeal membrane disintegrates to establish continuity between the two cavities.
As the oral cavity emerges, it includes the stomodeum and foregut and 2 important endocrine glands develop from its roof and floor. From the roof, an ectodermal lined pouch called Rathke’s pouch grows dorsally into the floor of the brain and gives rise to the anterior lobe of the pituitary gland. On the floor of the oral cavity, on the tongue, a second epithelial pouch develops and grows downward into the anterior neck to give rise to the thyroid gland. Both of these important endocrine glands develop from the oral tissue.
BRANCHIAL ARCHES The tissues bordering the oral pit inferiorly and laterally develop into five or six pairs of bars which form the lower part of the face and neck. These bars are termed branchial arches.
The first four branchial arches are well developed in humans. Only the first and second arches extend to the midline, and each arch is progressively smaller from first to the last. The mandibular branchial arch is the first to develop. It is located just below the stomatodeum. The hyoid is the second arch to develop. The IIIrd, IVth and Vth arches consist of paired bars of epithelial covered mesoderm which are divided in the midline by the developing heart.
BRANCHIAL GROOVES• The first branchial groove deepens to form the external auditory meatus.• The ectodermal membrane in the first groove persists and together with mesoderm and endoderm from adjacent first pharyngeal pouch, forms the tympanic membrane.• The external features of the 2nd,3rd and4th branchial grooves become obliterated by the overgrowth of the second branchial arch.• This overgrowth then provides the smooth contour of the neck.
PHARYNGEAL POUCHES The endodermal epithelium of the pharyngeal pouches differentiate into a variety of important organs. From the 1st pouch ,the middle ear and the Eustachian tube develop. From the 2nd, the palatine tonsils originate. From the 3rd pouch, the inferior parathyroid and the thymus arise. From the 4th pouch, the superior parathyroid gland forms. From the 5th pouch, the ultimobranchial body develops.
BRANCHIAL ARCH VASCULATURE Each of the 5 branchial arches contains a pair of blood vessels that conduct blood from the heart to the brain and to the posterior tissues through the arch tissues. These are called aortic arches.
The anterior right and left aortic arches develop first and, after a week, begin to disappear as more posterior arches develop. The most caudal arch vessels then enlarge and mature. The 5th arch vessels disappear next. The 3rd, 4th and 6th arch vessels do not disappear but are important in later functions. The 3rd arch vessels become the common carotid arteries which supply the neck, face and brain.
The 4th arch vessels become the dorsal aorta which supplies blood to the entire body. The vessels of the 6th arch supply blood to the lungs as pulmonary circulation. In an embryo at 4 weeks, the heart is ventral to the arches, and the blood passes dorsally to the brain and body. By the 5th week, the 1st and 2nd branchial arch vessels have disappeared, and then the blood supply to the face is carried out by the 3rd branchial artery which becomes the carotid artery.
SHIFT IN THE BLOOD SUPPLY OF FACE An important change in the human embryo takes place in the 7th prenatal week as the stapedial artery suddenly occludes and separates from the internal carotid artery; which discontinues its blood supply to the face and palatal tissues. Many of its terminal branches fuse with the peripheral branches of the external carotid. This results in the most unusual shift in the blood supply of the face, from the internal carotid to the external carotid artery. The timing of this shift is very important. The vessels begin to degenerate at one site and rapidly proliferate at another.
The 7th week is an important period of rapid growth expansion and fusion of the facial processes. The lip and palate are undergoing maximal developmental changes during this time. Thus, a vascular deficiency at this time may result in oxygen and nutritional deficiency which could result in cleft lip, cleft palate or both.
BRANCHIAL ARCH CARTILAGES The initial skeleton of the branchial arches develops from the mesenchymal tissue as cartilaginous bars. In the 1st arch, bilateral Meckel’s cartilages arise. The malleus and incus develop and ossify at the dorsal end of Meckels cartilage. The rest of the cartilage gradually disappears, leaving part of the perichondrium as the sphenomalleolar ligament (ant. Ligament of malleus) and part as the sphenomandibular ligament.
In the 2nd arch, Reichert’s cartilage develops. It gives rise to the stapes, styloid process, lesser horn and upper part of the body of the hyoid. The stylohyoid ligament is formed by the perichondrium at the site of disappearance of this 2nd arch cartilage. The 3rd arch cartilage forms the greater horn and lower part of the body of the hyoid. The 4th arch cartilage forms the thyroid cartilage. The 5th arch cartilage has no adult derivatives. The 6th arch cartilage forms the laryngeal cartilages.
MUSCULAR AND NEURAL DEVELOPMENT During the 5th week, myoblasts proliferate within the mandibular arch. By the 7th week, cells migrate and differentiate into the 4 muscles of mastication:lateral pterygoid, medial pterygoid, temporalis and masseter. The muscle cells within the hyoid arch and in the occipital myotomes undergo proliferation and migrate anteriorly toward the floor of the mouth to form muscles of the tongue. Muscle cells of the 3rd and 4th arch form the pharyngeal muscles : stylopharyngeus, cricothyroid, levator palatini and constrictor muscles of pharynx.
Nerves develop in conjunction with the developing muscle fibres. By the 7th week, the Vth nerve has entered the mandibular muscle mass, as has the VIIth nerve in the second arch mass. The trigeminal nerve (V) supplies sensory fibres to the mandible and maxilla and motor fibres to the muscles of mastication. The facial nerve (VII) follows the migration of the facial muscle mass from the neck onto the face. It also supplies the stylohyoid and stapedius muscles and posterior belly of digastric muscle. The glossopharyngeal nerve (IX) supplies the stylopharyngeus and the upper pharyngeal muscles. The vagus nerve (X) supplies the pharyngeal constrictor and laryngeal muscle
DEFECTS IN BRANCHIAL ARCH DEVELOPMENT Cervical Cysts and Fistulae: Caudal overgrowth of the second arch gradually covers the 2nd, 3rd and 4th branchial grooves. These grooves lose contact with the outside and temporarily form an ectoderm lined cavity, the cervical sinus, which should normally disappear.
Failure of complete obliteration of the cervical sinus results in a cervical cyst. If the cyst opens to the outside, a fistula develops. Branchial cysts or fistulae are found anywhere on the side of the neck along the anterior border of the SCM muscle. Another cause is incomplete caudal overgrowth of 2nd arch, which leaves an opening on the surface of the neck
DEVELOPMENT OF EARLY FACE The face develops during the 5th to 7th week of intrauterine life from 4 primordia that surround a central depression called the central pit. These include the frontal process (a single cranially located process), the 2 bilaterally located maxillary process, and the mandibular process derived from the first branchial arch.
DEVELOPMENT OF MANDIBLE The mandibular process appears initially as a partially divided bilateral structure but soon merges at the median line. This process will give rise to the mandible, the lower MERGED ME MANDIBLE part of the face and the body of the tongue.
By the 5th week, the nasal placodes develop bilaterally on the lower part of the frontonasal process where they border the oral cavity. At the margins of the placodes, mesenchyme proliferates and produces medial and lateral nasal processes thus transforming the placodes into nasal pits(nostrils). By the 6th week of IU life, The medial and lateral nasal processes appear as horse shoe shaped structures with the open end of the slit in contact with the oral cavity.
• The point of contact of the epithelial covered medial nasal and maxillary processes is termed the nasal fin.• This vertically positioned epithelial sheet under each nostril separates the medial nasal and maxillary processes; and when the fin disappears, the lip will fuse.
• On each side, the lateral nasal process is separated from the maxillary process by a groove called the nasolacrimal groove.• This groove will eventually disappear , but before it disappears, the epithelium at its depth will canalise , and form the nasolacrimal duct
DEVELOPMENT OF UPPER LIP AND MAXILLA During the 6th week, the 2 medial nasal processes merge in the midline to form the intermaxillary segment. This will give rise to the centre of the upper lip, the primary palate, and the part of the alveolar process carrying the incisor teeth. Each maxillary process grows medially and fuses, first with the lateral nasal processes and then with the medial nasal process. The medial and lateral nasal processes also fuses with each other ;thus closing the nasal pits to the stomatodeum.
The mesoderm of the lateral part of the lip is formed from the maxillary process. The overlying skin is derived from ectoderm of the same process. The failure of fusion of medial nasal process with the lateral nasal process leads to the formation of cleft lip.
DEVELOPMENT OF EYE The eyes develop during the 5th week. The first external sign of eye development is the appearance of the lens placodes between the maxillary and frontonasal processes at the lateral sides of the face.
The lens placode sinks below the surface and is eventually cut off from the surface ectoderm. The developing eyeball now presents as a bulge facing laterally. With the narrowing of the frontonasal process, they come to face forwards. The eyelids are derived from folds of ectoderm that are formed above and below the eyes, and by mesoderm enclosed within the folds.
DEVELOPMENT OF EAR The external ear is formed around the dorsal part of the 1st ectodermal cleft. A series of mesodermal thickenings appear on the mandibular and hyoid arches where they adjoin this cleft. The pinna is formed by fusion of these thickenings. When first formed the pinna lies caudal to the developing jaw. It is pushed upwards and backwards due to later enlargement of the mandibular process
DEVELOPMENTAL DEFECTS OF EAR Familial expansile osteolysis Malleus/incus fixation Absence of the long process of the incus Congenital fixation of stapes (stapes anchored to oval window) Failure of annular ligament development Cholesteatoma Congenital preauricular sinus.
DEVELOPMENT OF TONGUE The tongue is composed of the body which is the movable oral part and the posterior (attached) base or pharyngeal part. The tongue develops from the tissues of the 1st, 2nd and 3rd branchial arches and from the occipital myotomes. The body of the tongue develops from 3 elevations on the ventromedial aspect of the 1st arch: a tuberculum impar and paired lateral lingual swellings. These lateral lingual swellings rapidly enlarge, merge with each other , and overgrow the tuberculum impar to form the oral part of the tongue. A U-shaped sulcus develops in front and on both sides of this oral part, which allows it to be free and highly mobile except at the region of the frenum lingulae.
The base of the tongue develops mainly from the 3rd branchial arch. Initially, it is indicated by 2 midline elevations that appear caudal to the tuberculum impar. These are the copula of the 2nd arches and the large hypobranchial eminence of the 3rd and 4th arches. Later the hypobranchial eminence overgrows the 2nd branchial arches to become continuous with the body of the tongue. The site of union between the base and body of the tongue is delineated by a V-shaped groove called sulcus terminalis. The occipital myotomes migrate anteriorly into the tongue during the 5th to 7th weeks. Later, various types of papillae differentiate in the dorsal mucosa of the body of the tongue, whereas lymphatic tissue develop into the base of the tongue.
INNERVATION OF TONGUE As the occipital muscle masses migrate anteriorly, the IXth and XIIth nerves are carried along into the tongue. The Vth nerve supplies sensory fibres to the body or anterior 2/3rds of the tongue. The VIIth nerve supplies the taste fibres to the same part. The IXth nerve supplies sensory taste fibres to the posterior 1/3rd The hypoglossal nerve supplies the intrinsic muscles (longitudinal, vertical and transverse) and the extrinsic muscles (styloglossus, hyoglossus and genioglossus).
DEVELOPMENTAL DEFECTS OF TONGUE Aglossia Microglossia Macroglossia Ankyloglossia Cleft tongue Fissured tongue MACROGLOSSIA Median rhomboid glossitis Benign migratory glossitis
DEVELOPMENT OF THYROID In the 4th week, the thyroid gland develops as a depression and epithelial thickening in the floor of the pharynx. This appears at a point between the body and base of the tongue called the foramen caecum. From this point, the thyroid primordium descends in the neck as a bilobed diverticulum to reach in front of the trachea in the 7th week. During this migration, the gland remains connected to the floor of the oral cavity by an epithelial cord or duct, the thyroglossal duct which later becomes a cord of cells. The foramen caecum remains at the site of origin. The thyroid gland begins to function at the beginning of the 3rd month when colloid containing follicles appear.
DEVELOPMENTAL DEFECTS OF THYROID Thyroglossal cyst and Fistula: Cysts and fistulae found along the midline of the neck usually develop from remnants of thyroglossal duct. Generally, thyroglossal cysts maybe found at any point along the course of the thyroglossal duct but it is usually found at the level of the hyoid bone and the thyroid cartilage.
DEVELOPMENT OF SALIVARY GLANDS The major salivary glands (parotid, submandibular and sublingual) begin development during 6th to 8th week. The parotid develops in the lateral aspects of the stomodeum, and the submandibular and sublingual develop in the floor of the stomodeum.
Each gland develops through growth from a bud of oral epithelium into the underlying mesenchyme. The epithelial buds differentiate into extensive system of solid cords of cells which later form lumen and become ducts. Minor salivary glands develop during the 3rd prenatal month. They remain as separate acini scattered in the connective tissue underlying the oral mucosa. Failure of canalisation of ducts before acinar secretion begins results in retention cysts.
DEVELOPMENTAL DEFECTS OF SALIVARY GLANDS Agenesis Aplasia Atresia of ducts Xerostomia Developmental lingual salivary gland depression. Anterior lingual depression
DEVELOPMENT OF PALATE By the 6th week of development, the primitive nasal cavities are separated by a primitive nasal septum and partitioned from the stomodeum by a primary palate. The formation of secondary palate commences between 7 and 8 weeks and is completed around the 3rd month of gestation. Three outgrowths appear in the oral cavity: the nasal septum grows downwards from the frontonasal process along the midline, and 2 palatal shelves or processes , one from each side, extend from maxillary process towards the midline. The shelves are directed first downward on each side of the tongue.
After the 7th week of development, the tongue is withdrawn from between the shelves, which now elevate and fuse with each other above the tongue and with the primary palate. The septum and 2 shelves converge and fuse along the midline, thus separating the oronasal cavity into oral and nasal cavities.
For the fusion of palatine shelves to occur, elimination of the epithelial covering of the shelves is necessary. To achieve this fusion, DNA synthesis ceases within the epithelium some 24 to 36 hours before the epithelial contact. Surface epithelial cells are sloughed off as they undergo physiologic cell death to expose the basal epithelial cells. These cells have the carbohydrate rich surface coat that permits rapid adhesion and the formation of the junctions to achieve fusion of the processes.
A midline seam is thus formed of two layers of the epithelial cells. This midline must be removed to permit ectomesenchymal continuity between the fused process. The growth of the seam fails to keep pace with the palatal growth so that the seam first thins and then breaks down into discrete islands of epithelial cells. The basal lamina surrounding these cells is lost and the epithelial cells transforms into mesenchymal cells.
PALATAL SHELF ELEVATION This process has been presumed to take place rapidly, about as fast as the act of swallowing, as it has never been precisely recorded. Several mechanisms have been proposed to account for the movement of the palatal shelves from vertical to the horizontal position. The closure of the secondary palate may involve an intrinsic force in the palatine shelves the nature of which has not been determined yet. The extrinsic forces derived from the tongue and jaw movements may be responsible for this. The high content of glycosaminoglycans , which attract water and make the shelves turgid, has also been suggested.
DEVELOPMENTAL DEFECT OF PALATE Cleft Lip: Can be unilateral, bilateral and can vary from a notch in the vermillion border to a cleft extending into the floor of the nostril. Cleft palate: Less common than cleft lip. It maybe due to lack of growth or failure of fusion between the median and lateral palatine processes and the nasal septum or it maybe due to initial fusion with interruption of growth at any point along its course. It may also be due to interference with elevation of palatal shelves.
DEVELOPMENT OF TEETH In humans, 20 primary and 32 permanent teeth develop from the interaction of oral epithelial cells and the underlying mesenchymal cells. The tooth germ is derived from the dental lamina which is formed from the ectodermal cells of the first branchial arch. The dental lamina connects the tooth germ to the outer ectodermal layer. The tooth germs gets organized into 3 parts: the enamel organ, the dental follicle and the dental papilla.
STAGES OF TOOTH DEVELOPMENT1) THE BUD STAGE: The bud stage is characterized by the appearance of a tooth bud without a clear arrangement of cells. The stage technically begins once epithelial cells proliferate into the ectomesenchyme. Typically, this occurs when the fetus is around 6 weeks old.
2) THE CAP STAGE: The first signs of an arrangement of cells in the tooth bud occur in the cap stage. A small group of ectomesenchymal cells stops producing extracellular substances, which results in an aggregation of these cells called the dental papilla. At this point, the tooth bud grows around the ectomesenchymal aggregation, taking on the appearance of a cap, and becomes the enamel (or dental) organ.
3) THE BELL STAGE: The tooth histodifferentiation and morphodifferentiation takes place in bell stage. The dental organ is bell-shaped during this stage, and the majority of its cells are called stellate reticulum because of star- shaped appearance. Cuboidal cells on the periphery of the dental organ here are known as outer enamel epithelium. The columnar cells of the enamel organ adjacent to the dental papilla are known as inner enamel epithelium. The cells between the inner enamel epithelium and the stellate reticulum form a layer known as the stratum intermedium. The rim of the dental organ where the outer and inner enamel epithelium join is called the cervical loop.
4) ADVANCED BELL STAGE: Hard tissues, including enamel and dentin, develop during this stage of tooth development. In prior stages, all of the inner enamel epithelium cells were dividing to increase the overall size of the tooth bud, but rapid dividing stops during this stage at the location where the cusps of the teeth form. The first mineralized hard tissues(enamel and dentin) form at this location. At the same time, the inner enamel epithelial cells change in shape from cuboidal to columnar.
MINERALIZATION Mineralization is the process of deposition of the matrix of the hard dental structures, also called as the appositional growth. It is characterized by regular and rhythmic deposition of the extracellular matrix, which is itself incapable of further growth. The process of formation of enamel is known as amelogenesis, and that of dentin is known as dentinogenesis. The development of enamel involves two processes: organic matrix formation and mineralization.
The ameloblasts begin their secretory activity when a small amount of dentin has been laid down. The surface of ameloblasts facing the enamel is not smooth but possesses projections which in corporate into the enamel matrix termed as tomes processes. The mineralization of enamel matrix occurs in two stages. In the first stage interprismatic substances are laid down, and in the second stage gradual completion of the process occurs by total mineralization of the structure.
During the formation of dentin, the odontoblasts differentiate from a ovoid to a columnar shape. One or several processes arise from its apical end in contact with the basal lamina. The cell recedes apically and deposits the dentinal matrix gradually, and the several processes join into one. The single process is termed dentinal tubule. The mineralization occurs in the form of very fine layers of hydroxyapatite deposited in the ground substance. The crystals are arranged in an orderly fashion, with their long axis paralleling the fibril long axis.
FORMATION OF ROOT The development of root begins after enamel and dentin formation has reached the future cementoenamel junction. The enamel organ forms the ‘Hertwig Epithelial Root Sheath’ which moulds the shape of the roots and initiates radicular dentin formation. The differentiation of odontoblasts and the formation of dentin follows the lengthening of the root sheath.
DEVELOPMENT OF ADJACENT CONNECTIVE TISSUES1) CEMENTUM: Cementoblasts are the cells responsible for cementogenesis. Two types of cementum is formed: cellular and acellular. The cementoblasts secrete fine collagen fibrils along the root surface at right angles before migrating away from the tooth. As the cementoblasts move, more collagen is deposited to lengthen and thicken the bundles of fibers.2)PERIODONTAL LIGAMENT: Cells from the dental follicle give rise to the periodontal ligament. The fibroblasts in the dental follicle secrete collagen, which interacts with fibers on the surfaces of adjacent bone and cementum.
3) GINGIVA: Hemidesmosomes form between the gingival epithelium and the tooth. These are responsible for the primary epithelial attachment. During eruption, junctional epithelium forms from the reduced enamel epithelium. This epithelium divides rapidly, resulting in increased size of the junctional epithelial layer and the isolation of the remnants of ameloblasts devoiding them from any source of nutrition. As the ameloblasts degenerate, a gingival sulcus is created.4) ALVEOLAR BONE: Throughout the body, cells that form bone are called osteoblasts. These osteoblast cells form from the dental follicle. Similar to the formation of cementum, collagen fibers are created on the surface nearest the tooth, and deposit the fibres to form the alveolar bone.
I will praise thee: for I am fearfully and wonderfully made. -Psalm CXXXIX 14
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